Possible mechanisms of β-adrenoceptor-mediated relaxation induced by noradrenaline in guinea pig taenia caecum

The mechanisms of the β-adrenoceptor-mediated relaxation induced by noradrenaline in guinea pig taenia caecum were investigated. Noradrenaline caused graded relaxation of this preparation. However, the concentration-response curves for noradrenaline were unaffected by propranolol (∼ 10 −5 M) or phentolamine (∼ 10 −5 M). The responses to noradrenaline were antagonized in a concentration-dependent manner by bupranolol, and Schild plots of the data revealed a pA 2 value of 5.53. Also, bupranolol antagonized responses to isoprenaline, and Schild plots of the data revealed the pA 2 value to be 8.53. Noradrenaline significantly increased the cyclic AMP level in this preparation. Bupranolol (10 −4 M) significantly decreased the cyclic AMP response elicited by noradrenaline, whereas propranolol (10 −5 M) produced no effect. These results suggest that the relaxant response to noradrenaline in guinea pig taenia caecum is mainly mediated by β 3-adrenoceptors (or atypical β-adrenoceptors) and that in guinea pig taenia caecum noradrenaline behaves as a β 3-selective adrenoceptor agonist.